Biochemical Oxygen Demand

BOD is the amount of oxygen required by microorganisms to decompose organic matter dissolved in water. It becomes an important parameter of environmental science in defining the quality of the water and the health condition of aquatic ecosystems. In a progressively polluted world, the BOD will act as an indicator of the extent of organic pollution within the water and has serious consequences for aquatic life. In the case of organic matter, like plant debris or wastewater, this raises the demand for oxygen, which microorganisms break down from such matter in a water body. One depletes the dissolved oxygen to such a level that it might suffocate fish and other aquatic animals as a result of hypoxic conditions. BOD thus offers a basis upon which scientists and other environmentalists can project the probable effect the pollutants will have on ecosystems, and in applying strategies for water treatment in order to ensure the sustainability of the water resources.

It is critically directly proportional to the health of an aquatic ecosystem and to the efficiency of the process of treating wastewater. High values of BOD express large amounts of organic matter and hence indicate that the water is polluted and probably harmful to aquatic life. The measurement of BOD involves incubating a sample of water in the dark to prevent photosynthesis, measuring the change in the initial and final dissolved oxygen level, and finding the difference to get the oxygen consumed by microorganisms. In general, temperature, pH, presence of specific microorganisms, and class of organic and inorganic matters in the water impact BOD.

Understanding Biochemical Oxygen Demand

• The milligrams of dissolved oxygen used by microorganisms in breaking down the organic matter present in a water sample in a five-day period usually at 20°C.
• BOD is the most important and essential parameter to determine the organic load of pollution on receiving waterbodies. Higher values indicate a large amount of organic matter; this may signify severe water pollution.
- Measurement of BOD involves incubating a water sample in the dark and measuring the initial and final DO levels before finding the difference, which will yield the oxygen consumed by the micro-organisms.
- The parameters that influence the value of BOD are temperature, pH, type of microorganisms, and kind of organic and inorganic materials in water.

Dissolved oxygen(DO) is essential for sustaining animal and plant life in any aquatic system. If the DO level in water decreases, the aquatic organisms may not survive. The wastes such as domestic, industrial and biodegradable organic compounds are oxygen-demanding wastes. These are decomposed by the bacterial population which in turn decreases the oxygen from water.
BOD is a measure of the oxygen utilized by microorganisms during the oxidation of organic materials. It is the most widely known measure for assessing the water pollution of a given organic waste. BOD is directly proportional to the amount of organic waste that is to be broken down.
The determination of the BOD of a sample of water requires 20-30 days for the complete decomposition of organic waste present in the sample. This is too long a time for obtaining data. Therefore, the time has been fixed as five days, i.e., BOD is the amount of oxygen consumed in five days. The measurement is done at 20^oC. The sample of water is saturated with oxygen. It is then incubated for five days at 20^oC. Micro-organisms utilize oxygen to oxidize waste during this period. The remaining oxygen is measured and subtracted from the amount of oxygen originally present to get the BOD value. It is expressed in ppm.
BOD is a real measure of water quality. Clean water has a BOD value of less than 5 ppm. The polluted river water could have a BOD value of 17ppm or more. The untreated municipal sewage has a BOD value of 100-400 ppm.

Parameters and Forms of Biochemical Oxygen Demand

There are a few types of BOD, all of which have different purposes in the area of environmental science.
Five-Day BOD: This is the standard measurement taken for five days, and this one is widely adopted to classify or measure the degree of water pollution.
Ultimate BOD: It shows the total oxygen demand by organic material in a sample for a long period and is hence applied for long-term assessment purposes.
• Carbonaceous BOD: The amount of oxygen required for the decomposition of only carbon-containing organic matter by microorganisms, i.e., excluding nitrogenous contributions.
• Nitrogenous BOD: That contribution to the oxygen demand made by nitrogenous compounds, mainly because of ammonia, which can equally contribute towards degradation in water quality.

How Biochemical Oxygen Demand is useful and its Applications in Real Life

BOD has relevant implications for environment management and public health with regard to wastewater treatment and regulatory compliance.
- In that respect, BOD measurements become very important in wastewater treatment since they express the amount of organic wastes that shall be broken down during such processes, and whether the treated water is safe for release into natural water bodies.
- It provides the regulatory agencies' basis of allowable values of BOD with respect to the industrial discharges for the protection of aquatic ecosystems; hence industries must treat their wastewaters prior to discharge to a water body to ensure low discharged BOD values.
• Long-term monitoring for BOD can identify sources of impairment, focus restoration efforts, and further inform policy strategies likely to improve the quality of the water.
• Academic researchers use BOD data for understanding the self-purification capacity of rivers and streams and modeling water quality as a function of the suite of different pollutant types impacting on aquatic ecosystems.

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Some Solved Examples

Example 1 Biochemical Oxygen Demand(BOD) is the amount of oxygen required (in ppm):

1)For the photochemical breakdown of waste present in 1 m³ volume of a water body

2)by anaerobic bacteria to break down inorganic waste present in a water body.

3) (correct)by bacteria to break down organic waste in a certain volume of a water sample

4)for sustaining life in a water body

Solution

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand (BOD).

Hence, the answer is the option (3).

Example 2 Given below are two statements :

Statement I: The value of the parameter " Biochemical Oxygen Demand (BOD)" is important for the survival of aquatic life.

Statement II: The optimum value of BOD is 6.5 ppm.

In the light of the above statements, choose the most appropriate answer from the options given below:

1) (correct)Statement I is true but statement II is false

2)Both statement I and statement II are false

3)Statement I is false but statement II is true

4)Both Statement I and Statement II are true

Solution

Statement I is correct.

Statement II is incorrect.

If too much organic matter is added to water, all the available oxygen is used up. This causes oxygen-dependent aquatic life to die. The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water is called Biochemical Oxygen Demand (BOD). The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically. Clean water would have a BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more.

Hence, the answer is the option (1).

Question:

Example 3 The measured BOD values for four different water samples (A-D) are as follows: A= 3 ppm; B=18 ppm; C=21 ppm D=4 ppm. The water samples which can be called as highly polluted with organic wastes, are

1)A and B

2)A and D

3) (correct)B and C

4)B and D

Solution

As we have learned,

Highly polluted water has a BOD of more than 17 ppm.

Thus, water samples B (18 ppm) and C (21 ppm) are highly polluted.

Hence, the answer is the option (3).

Example 4 Given below are two statements :
Statement I: In polluted water values of both dissolved oxygen and BOD are very low.

Statement II: Eutrophication results in a decrease in the amount of dissolved oxygen.
In the light of the above statements, choose the most appropriate answer from the options given below :

1)Both Statement I and Statement II are true

2)Both Statement I and Statement II are false

3)Statement I is true but Statement II is false

4) (correct)Statement I is false but Statement II is true

Solution

Eutrophication is a process due to which excessive growth of weeds in water bodies takes place leading to an increase in pollution levels. Eutrophication leads to a decrease in the amount of dissolved oxygen in water bodies. As the amount of dissolved oxygen decreases, the value of BOD [Biological oxygen demand] increases.
So statement I is false and statement II is true.
Hence, the answer is the option (4).

Example 5 The water having more dissolved O₂ is :

1)Boiling water

2)Water at $80^{\circ} \mathrm{C}$

3)Polluted water

4) (correct)Water at $4^{\circ} \mathrm{C}$

Solution

The solubility of gases in water decreases with the increase in temperature.

Thus, more dissolved O₂ will be present in water at $4^{\circ} \mathrm{C}$

Hence, the answer is the option (4).

Summary

Lastly, BOD is the basic concept meaning biochemical oxygen demand, which is the amount of oxygen needed for microorganisms to break down organic matter in the water. The paper tried to define, explain, and expound on the meaning, types, and factors affecting BOD in a way that would underline its role as a pointer or indicator of water quality and pollution. We shared with them practical applications of BOD determination in the realm of wastewater treatment, regulatory compliance, and academic research. We underpinned the importance of BOD in having a healthy aquatic ecosystem. Fifth, understanding BOD is key to devising intervention measures against water pollution and ensuring the sustainability of our lifeline water resources. Keeping of the BOD levels under control will protect aquatic life and public health while sustaining the responsible use of our water resources for present and future generations.